In a conventional internal combustion engine for automotive vehicles, a fuel and air mixture is provided in correct proportions and a spark is used for igniting the air/fuel mixture. The spark is timed in relation to the position of the pistons in the engine cylinders to generate maximum torque while avoiding abnormal combustion of the air/fuel mixture. The variables that influence the optimum spark timing for any given operating condition include engine speed, manifold pressure, coolant temperature, intake air temperature, ambient pressure, and fuel octane. The correct spark timing based upon the instantaneous values for these variables is stored in a look-up table in the memory of a microprocessor, which forms a part of the electronic engine control system.
The engine control system obtains readings from various sensors whose signals are a measure of the combustion variables and generates an appropriate address to the look-up table in ROM. The control system then computes the correct spark advance for each cylinder.
Generally, advancing the spark toward top dead center for each cylinder increases the torque until a point at which maximum torque is achieved. If the spark is advanced too far, abnormal combustion known as knocking or pre-detonation will occur. This is characterized by an abnormally rapid rise in cylinder pressure during combustion. That rapid rise in pressure is followed by pressure oscillations, the frequency of which is specific to a given engine configuration and cylinder dimension. Typically, the frequency is in a relatively narrow range of only a few kilohertz.
A relatively low energy level of knock arguably is beneficial to engine performance, but audible knock may result in vehicle operator dissatisfaction, and excessive knock can damage the engine. A typical control strategy will distinguish between acceptable and unacceptable levels of knock. The engine control will advance the spark until the knock level becomes unacceptable. This is determined empirically. At that point, the control system will reduce the spark advance until an acceptable level of knock is achieved.
A control system of this type requires a knock sensor that responds to engine vibration energy and functions in the spectrum of rapid cylinder pressure oscillations. Accurate control of knock permits the engine to be calibrated closer to the optimum ignition timing.
The degree of knock depends upon the amount of energy available and the rate of combustion of the end gas. Factors that have an effect on the degree of knock include cylinder temperature, volumetric efficiency, residual burned charge, air/fuel ratio, spark timing, octane, homogeneity of the air/fuel mixture, cylinder geometry, compression ratio, and the amount of unburned fuel in the end gas when it auto-ignites.
Since many of these variables change from cycle to cycle and from cylinder to cylinder, the level of knock also changes from cycle to cycle and from cylinder to cylinder. Therefore, knocking is a random phenomenon, and any variable that affects the combustion process or changes the mass, pressure, temperature, or composition of the end gas contributes to knock intensity and rate of occurrence.
We are aware of knock detection systems that include audio transducers for converting audio signals indicative of abnormal engine combustion into an output voltage that can be used by a microprocessor in controlling engine timing to eliminate knock. Examples of these prior art devices are described in U.S. Pat. Nos. 4,667,636 and 4,761,992. In the system of the '636 patent, an audio transducer is mounted adjacent to a cylinder in a multi-cylinder internal combustion engine. The cylinder that is selected is one that is more prone to knocking than the other cylinders. The signal that is obtained from the transducer is filtered and sampled. The voltage amplitudes of several samples are compared by a comparator circuit. If a sample that is measured at an instant later than a sample measured earlier in the combustion cycle is greater in magnitude by a predetermined amount, it is assumed that detonation is occurring and an appropriate signal is distributed to a fuel enrichment control or to a spark retard control, or to both, until the detonation is eliminated.
In the control system of the '992 patent, an audio transducer is used to sample a signal that includes a background noise portion and a portion that represents detonation. The portion of the signal that represents background noise is used to develop a bias for the gain of a control transistor. A detonation threshold detector responds to a predetermined increase in the amplitude of the portion of the signal voltage that represents detonation above the value that represents background noise and then develops an output signal that is used by the microprocessor to adjust spark timing or fuel supply.